1,3-butadiene is a simple conjugated diene. It is a product of the petrochemical industry, used as a monomer starting material for the preparation of various polymers, including synthetic rubbers. 1,3-butadiene is often produced commercially by one of three processes: steam cracking of paraffinic hydrocarbons; catalytic dehydrogenation of n-Butane and n-Butene; or oxidative dehydrogenation of n-Butene. The product of these processes is known as crude 1,3-butadiene, which contains propyne, 1,2-butadiene, and C5 hydrocarbons, as well as other compounds. This crude 1,3-butadiene can then be further purified using a distillation process. Purification methods of crude 1,3-butadiene are commonly known, and typically involve a series of two distillation columns. In such methods, crude 1,3-butadiene is fed into the first distillation column. Propyne can be removed as an overhead gas from the top of the first column, while the bottoms, containing 1,3-butadiene, 1,2-butadiene, and C5 hydrocarbons are sent to a second butadiene distillation column. The purified 1,3-butadiene product is withdrawn as a liquid from the top of the second column, and the 1,2-butadiene and C5 hydrocarbons are removed as a liquid from the bottom of the second column.
One common problem found in certain butadiene extraction systems is the formation of butadiene polymer, commonly identified as popcorn polymer. Polymerization occurs where monomers react spontaneously to form polymer chains. Popcorn polymer can form in both the liquid and vapor phase, but is most likely to form in the vapor spaces where the concentrations of olefins and temperature are both very high. Formation and growth of popcorn polymer is often accelerated by external factors such as oxygen and iron oxide (a by-product of corrosion). The rapid expansion rate of popcorn polymer creates significant concerns for processing equipment. If uncontrolled, the growth of popcorn polymer may result in the clogging of processing equipment or piping, and may even lead to equipment rupture or fracture due to mechanical pressure.
Certain methods for inhibiting polymer formation for reactive monomers are known in the art. G.B. Patent No. 1,472,859 discloses the introduction of nitric oxide prior to distillation of C4 to C5 diolefins in order to reduce fouling by “rubber-like” deposits, i.e., polymer. This method can include purging the nitric oxide from the unit prior to its use. U.S. Pat. Nos. 4,754,058, 4,338,162, and G.B. Patent No. 798,347 disclose methods to inhibit polymerization by adding nitric oxide during distillation of reactive monomers. U.S. Pat. No. 4,404,413 discloses treatment of butadiene with carbon disulfide or elemental phosphorous. U.S. Pat. No. 5,345,030 discloses the use of sulfur-containing compounds as inhibitors of the polymerization of olefinically unsaturated monomers. U.S. Pat. No. 6,686,422 discloses the use of nitroxides to inhibit polymerization of olefins. U.S. Pat. No. 4,956,020 discloses the use of nitroso compounds to prevent popcorn polymer formation. U.S. Pat. No. 4,941,926 discloses the treatment of an olefin production apparatus with acetic acid-based compounds to prevent popcorn polymer formation.
A drawback of certain methods for inhibiting popcorn polymerization of butadiene is that the inhibitors are only effective in solution phase due to their high molecular weight. Formation of popcorn polymer however can be observed in the gaseous phase of a butadiene unit. There is a need for methods for inhibiting popcorn polymerization which are effective in the vapor phase of a butadiene distillation unit.